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Sourav Poddar

Biogas is a methane-rich gas produced from the breakdown of organic matter by microorganisms in an oxygen-free environment. It is a type of biofuel produced through anaerobic digestion or fermentation of biodegradable materials such as manure, sewage, municipal waste, and green waste. Anaerobic digestion is a multistage process where organic material is broken down into methane and carbon dioxide. Biogas has several advantages including producing renewable energy, reducing greenhouse gas emissions, and generating fertilizer. Examples of biogas applications include small-scale household digesters and large municipal waste treatment plants.

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Biogas Dr. Sourav Poddar Department of Chemical Engineering National Institute of Technology, Tiruchirappalli Tamil Nadu
Module - 2.pdf
What is biogas? • Biogas is a methane rich flammable gas that results from the decomposition of organic waste material • Biogas is produced by anaerobic digestion or fermentation of biodegradable materials such as biomass, manure, sewage, municipal waste, green waste, plant material and energy crops. • Biogas also called as ‘Marsh gas’ • Biogas is a type of biofuel. • This type of biogas comprises primarily methane and carbon dioxide
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Factors Influencing Biogas Composition
Advantages of biogas • Production of large amount of methane gas (ambient temperature storage) • Production of free flowing thick sludge • Odorless sludge • Sludge can be used as fertilizer and soil conditioner • Sanitary way for human and animal waste disposal • Conservation of scarce resources like wood
Disadvantages • Explosion chances • High capital lost • Incorrect handling of liquid sludge causes pollution • Requires control and maintenance • Needs proper condition • Use as a fuel requires removal of CO2 and H2S
Factors affecting yield and production of biogas Many factors affecting the fermentation process of organic substances under anaerobic condition are, • The quantity and nature of organic matter • The temperature • Acidity and alkanity (PH value) of substrate • The flow and dilution of material
GENERAL FEATURES OF BIOGAS
Biochemical reactions C 48.46 H 6.75 O 37.47 N 7.30 S 0.02 C16h27o9n2 + (27/2) h2 (23/2) ch4 + (9/2) co2 + h2 + 2 nh3 2C403h675o234n52s + (627) h2 (572) ch4 + (234) co2 + 104 nh3 + 2h2s
Liquefaction by hydrolytic enzymes • Complex organic matter is degraded to basic structure by hydraulic bacteria. ➢Protein - Polypeptide and Amino Acid ➢Fat -Glycerin and Fatty Acid ➢Amylose - Monosacride and Polysacride
Acid Formation • Also called the acidogenesis • Simple organic matters are converted into acetic acid, H2 and CO2 • Acting bacteria in this process are called hydrogen producing bacteria and acid-producing bacteria.
Methane Production • Methanogenesis • In this process, acetic acid, H2, CO2, are converted into CH4. • Methane-producing bacteria have strict pH requirement and low adaptability to temperature. • Methanococcus jannaschii, Methanobacterium thermoautotrophicum
Flow chart of anaerobic digestion
Modes of operation • Batch • Semi-continuous – regular feeding of digester and decrease of organic matter at intervals • Continuous – for liquid waste treatment
Continuous-fed System • Suited for large-scale manure substrate bioreactor. • Steady biogas production can be expected. • May require auxiliary equipments. • Requires high liquid content. • Temperature, loading rate, and solid content need to be carefully monitored.
Batch-fed System • The simplest design. • Low cost. • The feedstock is loaded one batch at time. • Irregular biogas production. • Can operate on high solid content. • Requires manual labor.
Types of digesters A typical biogas system consists of the following components: • (1) Manure collection • (2) Anaerobic digester • (3) Effluent storage • (4) Gas handling • (5) Gas use. TWO MAIN TYPES: 1. Digesters utilizing dispersed growth of bacteria 2. Digesters utilizing attached growth of bacteria
DISPERSED GROWTH DIGESTERS 1. Fixed dome digester 2. Floating gas holder digester (Indian) 3. Plug flow digester (horizontal displacement) 4. Bag digester (Taiwan and Korea) 5. Separate gas holder digester 6. Conventional digester
Fixed dome digester
Floating gas holder digester
Bag Biodigester
1. Anaerobic filter 2. Up-flow anaerobic sludge blanket (UABSR) Attached growth digesters
Anaerobic filter
UASBR Up-flow anaerobic sludge blanket
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Agricultural residue Agriculture residue Food waste Food processing waste Food processing waste Municipal waste
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3075 m3/day
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Materials and methods
Experimental set up
Results
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What is Anaerobic Digestion ? Degradation of organic material by bacteria. In the absence of air (anaerobic). Four stages: •Hydrolisis • Cleavage of a chemical compound through the reaction with water. • Insoluble complex molecules are bracken down to short sugars, fatty acids and amino acids. •Fermentation (Acidogenesis) • Products from hydrolysis are transformed into organic acids, alcohols, carbon dioxide (CO2), hydrogen (H) and ammonia (NH3). •Acetogenesis • Organic acids and alcohols are converted into hydrogen (H2), carbon dioxide (CO2) and acetic acid (CH3COOH). Therefore, oxygen is consumed and anaerobic conditions are created •Methanogenesis • Methanogenic bacteria (methanogenesis), transform the acetic acid, carbon dioxide and hydrogen into biogas. 1. Concept
What is Anaerobic Digestion ? 1. Concept D. SPUHLER (2010)
1. Concept What is Anaerobic Digestion ? Source: http://water.me.vccs.edu/courses/ENV149/changes/Feat11_picII- 1.jpg [Accessed: 02.06.2010]
What is Biogas ? •Biogas is a mixture of methane and carbon dioxide. •The properties of biogas are similar to the ones of natural gas. •Biogas is the common name for the mixture of gases released from anaerobic digestion. •Typically biogas is composed of: •Methane is the valuable part of the biogas. Biogas that contains about 60 to 70 % of CH4 has a calorific value of about 6 kWh/m3 what corresponds to about half an L of diesel oil. 1. Concept Sources: YADAV & HESSE (1981); FAO (1996); PIPOLI (2005); GTZ (2009 Methane (CH4) 50 to 75 % Carbon Dioxide (CO2) 25 to 50 % Hydrogen (H) 5 to 10 % Nitrogen (N2) 1 to 2 % Hydrogen sulphide (H2S) Traces Source: MUENCH (2008)
Anaerobic Digestion of Green Waste 1. Concept D. SPUHLER (2010), Pictures from: www.kristianstad.se/; http://www.newseedadvisors.com/2009/09/10/invest/; http://www.hydroharrys.com/hydroharrys_about_fertilizer.php; http://www.mytinyplot.co.uk/advice/the-art-of-composting/ www.clker.com [Accessed: 02.06.2010] Green Waste Coocking Lightning Electricity Fuel Biogas Heating Fertiliser Agriculture
Anaerobic Digestion of Green Waste: Small- scale 1. Concept Biogas Soil amendement Food Energy D. SPUHLER (2010). Pictures from: //gardening.ygoy.com/wp-content/uploads/2009/10/how-to-plant-a-decorative-vegetable- garden0.jpg; http://www.gardenplansireland.com/forum/about436.html; http://www.clker.com/ [Accessed: 06.06.2010]
Anaerobic Digestion of Green Waste: Large-scale 1. Concept Source: HOLLIGER (2008)
1. Concept Co-digestion of waste and manure Food production at farms Re-use of nutrients Re-use of energy The example of Kristianstad (Sweden) Green waste from households and industries
What is Green Waste? 1. Concept Kitchen refuses http://www.mytinyplot.co.uk/advice/the-art-of- composting/ [Accessed: 04.06.2010] http://www.ceroi.net/reports/dushanbe/eng/ waste.htm [Accessed: 04.06.2010] Organic fraction of municipal waste http://www.agro- resources.com/uploads/images /chocolate%20waste.jpg [Accessed: 04.06.2010] Refuses from the food industry http://www.titech.com/assets/x/50186 ?width=82 [Accessed: 04.06.2010] Some industrial wastes http://www.bawbawshire.vic.gov.au/Page/i mages/green-waste-grass.jpg [Accessed: 04.06.2010] Garden refuses Waste from agriculture http://planetgreen.discovery.com/hom e-garden/images/2009-04/organic- waste.jpg [Accessed: 04.06.2010] Market waste http://www.ducorwaste.org/images/Rally_Time_ Stockpike.jpg [Accessed: 04.06.2010] •Green waste is any kind refused material which is biodegradable and has a high fraction of organic matter, which can be transformed into biogas. Some examples…
Examples: Small-scale Biogas plants 1. Concept Biogas plant for cow dung, Padli village (India) Source: M. WRIGHT, Ashden Awards Source: M. WRIGHT, Ashden Awards Source: M. WRIGHT, Ashden Awards
Examples: Small-scale Biogas Plants 1. Concept Biogas lamp Source: M. WRIGHT, Ashden Awards Source: M. WRIGHT, Ashden Awards Adding greywater to the biogas reactor to optimise moisture conditionss
Examples Small-scale Biogas Plants 1. Concept The “Mudbooster” Plant Source: UNKNOWN
Examples Small-scale Biogas Plants 1. Concept Source: C. RIECK (2009) The manhole is filled with water to keep the clay sealing wet and gas tight. Gas leackage would be indicated by bubbles. Source: C. RIECK (2009) Wet clay is used to fit the concrete lid of the manhole gas-tight. Source: SuSanA Biogas outlet and manhole with remouvable cover from a underground biogas plant Installed by the NGO TED in Maseru, Lesotho (Susana)
2. How it can optimize SSWM D. SPUHLER (2010), adapted from: http://www.terranet.or.id/mitra/dewats/photo/masukan1256.jpg; http://www.borda-sea.org/modules/cjaycontent/index.php?id=6; http://whrefresh.com/wp- content/uploads/2010/01/potato_field.jpg; http://www.greenspec.co.uk/images/energy/CHP/chp2.gif]; http://peda.gov.in/eng/images/rural- biogas-plant_179.jpg; [Accessed: 30.05.2010], BPO (2006) and BUNNY (n.y.) Biogas plants transform traditional manure management; reducing CH4 and CO2 emission Biogas substitutes conventional energy sources, reducing reliance on fossil fuel and firewood (CO2) Digested sludge can substitute chemical fertiliser Biogas plants can contribute to sustainable sanitation
2. How the Digestion of Green Waste optimises SSWM Digested sludge can substitute chemical fertiliser and enhance food production Green Waste Coocking Lightning Electricity Fuel Biogas Heating Fertiliser Agriculture Sustainable development: • Improved health • Improved economy Anaerobic digestion is a promising answer to the soaring crises of municipal waste explosion and thus prevent the pollution of water sources and the evnvironment Biogas is an renewable energy and has the potential to replace other fuel sources. Biogas contributes to prevent and lower greenhouse gas emission.
Examples: Biogas Appliances 1. Concept Chang Mai K.P . Pravinjith M. Wafler Biogas generator Biogas rice cooker Biogas boiler Biogas lamps Biogas cooking stoves PBO (2006) Krämer (TBW) Source: UNKNOWN
The ARTI compact biogas plant •Developed in 2003 •ARTI = Appropriate Rural Technology Institute •2000 plants currently used in Maharashtra, India (WRAPAI 2009) •Some have been constructed in Tanzania (VOEGELI & LOHRI 2009) •Floating-drum design: •2 conventional polyethylene tanks (0.75 and 1 m3). (MUELLER 2007) •Standard plumber piping. •The smaller tank is the gasholder and the larger holds the mixture of decomposing feedstock and water •Inlet and an overflow •Overflow liquid is mixed with the feedstock and back recycled into the plant to maintain optimal moisture condition. (MUELLER 2007) •A pipe takes the biogas to a collection balloon or directly to the kitchen. Examples of Applications Source: HEEB (2009) Source: VOEGELI & LOHRI (2009)
The BIOTECH Plant •BIOTECH is a nodal agency of the Ministry of Non-Conventional Energy Sources in Kerala, South India. (MUELLER 2007) •Domestic plants: 1 m3 for a 3 to 5 member-family meets about 50 % of cooking needs. (MUELLER 2007) •Decentralised treatment of market waste, municipal solid waste or slaughterhouse waste: Biogas used for public street lightning and distributed into households Examples of Applications Source: HEEB (2009) Source: HEEB (2009) Source: HEEB (2009)
Source: HEEB (2009) The BIOTECH Plant •Schematic plan of a BIOTECH market level plant. a) Inlet tank for feedstock. b) Digester tank. c) Effluent tank. d) Effluent storage tank. e) Effluent pump. f) Gasholder drum. The drum is stabilized by a guide pole in the middle and is floating in a water jacket outside the digester. g) Biogas pipe. h) Gas Scrubber. i) Biogas generator j) Drainage connection for excess effluent. (HEEB 2009) Examples of Applications • Conventional floating-drum reactor • Liquids are mixed with incoming feedstock and re-circulated • A baffle in the middle increases solids retention
The KOMPOGAS Plant •Thermophilic dry digestion process developed in Switzerland •Organic wastes come from municipalities with source separation or from food industry •Horizontal plug-flow reactors •Propellers move the sludge trough the reactor and keep particles in suspension. (OSTREM 2004) •Retention time is 20 days. (MES et al. 2003) Examples of Applications Biogas is transformed in a CHP unit providing 100 % of the facility needs as well as additional electricity for sale. In some cases, the biogas is upgraded to natural gas standards for use in vehicles or input to the natural gas network. (OSTREM 2004) Both liquid and solid effluents are commercialised fertilisers
Large-scale Plant in Thailand for Municipal Waste •In Thailand, where the development of alternative sources is critical to energy sustainability as the government has set 2011 as the target date for 80 % of the nation’s total energy, representing an estimated 1,900 MW, to be generated from renewable energy sources. (MUELLER 2007) •This has given large rise to various large-scale biogas projects. The Rayong Municipality has constructed plant for the treatment of the organic fraction of the municipal solid waste (MSW) with a capacity of 60 tons of waste per day. •Two systems: •Digesters: converts waste to biogas and fertilizer •A biogas-fired cogeneration process (CHP) Examples of Applications MUELLER (2007) MUELLER (2007)
•Process: Wet fed-batch high-solids digestion •Feedstock: Organic MSW and refuses from the food industry. •Capacity: 60 tons per day •Output: 5800 tons organic fertilizer and electricity of about 5 million kWh.(MUELLER 2007) Large-scale Plant in Thailand for Municipal Waste Examples of Applications SourcE: MUELLER (2007)
Basics: Process Parameters The biogas yield depends on the process and the substrate. Substrate: •High COD (Chemical Oxygen Demand) = High potential of biogas generation Process: Anaerobic digestion = Biological system of bacteria Optimal conditions required that bacteria feel wealthy… •Temperature - Temperature is an important parameter. Mesophilic methane producing bacteria grow at an optimum temperature of 35oC the gas production rate drops very much when temperature is less than 10oC. •Performance •Retention time - The ratio of volume of slurry in the digester to the volume fed into and removed from it per day is called retention time. Thus a 20 liter digester is fed at 4 liters per day so that the volume of digester is constant the retention time is 5 days. The required retention time is normally 30 days for mesophilic (25-35oC) conditions. •pH • Wide range, but methanogenesis requires neutrality (6.5-7.5) (MES et al. 2003) • Multistage process for better pH and temperature control •Total solid (TS) and moisture • Wet digestion (TS < 20 %): easier to maintain, good fluidity • Dry digestion (TS > 20 %): sophisticated but safes space • Solids for digestion (organics) - Liquid for fluidity of slurry. • Optimal TScontent: 5 to 10%. (SASSE 1988; NIJAGUNA 2002) Design Principals
Basics: Process Parameters Anaerobic digestion = Biological system of bacteria Optimal conditions required that bacteria feel wealthy… • COD: Chemical oxygen demand: Methane production potential • Ratio of carbon to nitrogen in the waste water influent or C/N ratio is 30:1 and if nitrogen content in ammoniacal form is less the bacterial growth is affected and the process slows down. • Volumetric organic loading rate: This can be expressed as kg Vs per volume per day based on the % weight of organic matter added each day to the digester volume. • Digester loading rate % = (Per cent of organic matter in feed)/(Retention Time) Loading rate range is 0.7 to25 kg VS/ m3 / Day Design Principals
• Anaerobic digestion can transform almost any biodegradable waste into biogas (e.g. green waste). •The anaerobic treatment of organic solid waste is applicable everywhere where there is a need for biogas and waste treatment and the technical conditions allow the installation of a plant. •Small-scale (biogas generation for cooking and lightening) – low-cost and relatively low-tech: • Household-level • Community-level • Institutional-level •Large-scale – high-tech, requires expert design: • Industrial plant connected to the public power and heat grid. •Low-tech (un-heated plants), however, are only adapted to areas where temperature does not fall short of for any substantial length of time. Applicability
Basics: Daily manure yield for different cattle Design Principals Sources: OEKOTOP; WERNER et al. (1998)
Basics: Gas yields for different feedstocks Design Principals Sources: OEKOTOP; WERNER et al. (1998)
Basics: Biogas Guideline data Design Principals Adapted from WERNER et al. (1998); ISAT/GTZ (1999), Vol. I; MANG (2005) Suitable digesting temperature 20 to 35 °C Retention time 40 to 100 days Biogas energy 6kWh/m3 = 0.61 L diesel fuel Biogas generation 0.3 – 0.5 m3 gas/m3 digester volume per day Human yields 0.02 m3/person per day Cow yields 0.4 m3/Kg dung Gas requirement for cooking 0.3 to 0.9 m3/person per day Gas requirement for one lamp 0.1 to 0.15m3/h
Types of Digester: Bag or Rubber Balloon Biogas Plants Design Principals Batch mode: emptying once every few years Plug-flow reactor: the slurry moves through continuously much like a train a tunnel •Huge common plastic bag (e.g. PVC): sludge settles on the bottom and biogas is collected in the top. Gas is transported by the pressure from the elasticity of the balloon (can be enhanced by placing weights on the balloon). • Most simple design, easy and low-cost ( if material locally available) • Temperature enhanced when exposed to sun • Simple to clean but lifespan generally limited Plastic bag Layer of compacted backfill To reuse or further treatment (e.g. drying bed) Source: adapted from FAO (1996) Gas pipe Inlet Leveled surface Biogas accumulates in the top of the bag
Types of Digester: Bag or Rubber Balloon Biogas Plants Design Principals Source: http://www.habmigern2003.info/biogas/Baron-digester/Baron-digester.htm [Accessed: 02.06.2010] Underground plug- flow reactor bag biogas plant () and balloon biogas collection chamber (). (Philippines, Garry Baron)
Types of Digester: Fixed-dome Biogas Plants •Airtight underground reactor out of concrete or brick work (most often round), with a fixed (also airtight) dome in which gas is collected. Gas pressure is absorbed by the slurry which is displaced into a compensation tank. • Most widely disseminated • Long life-spam • Underground: safes space and protect from temperature changes • Construction must be supervised Design Principals
Types of Digester: Fixed-dome Biogas Plants Design Principals Removable cover Overflow tank / compensation chamber Seal Slurry Biogas accumulates in the dome Biogas collection Inlet Source: adapted from http://peda.gov.in/eng/images/rural-biogas-plant_179.jpg [Accessed: 02.06.2010] Fixed- dome
Source: K.P. PRAVVIJITH Source: K.P. PRAVVIJITH Source: K.P. PRAVVIJITH Source: K.P. PRAVVIJITH Types of Digester: Fixed-dome Biogas Plants Design Principals
Plastic dome Design Principals Any pit can be filled with organic waste and covered airtight with a plastic sheet in order to collect biogas Source: ISAT/GTZ (1999, Vol. I)
Floating-drum Biogas Plants 3. Design Principals Floating- drum Biogas Slurr y Inle t Outl et •Floating-drum plants consist of an underground digester and a moving gasholder (mostly of made out of steel). The gasholder floats either directly on the fermentation slurry or in a water jacket of its own. The gas is collected in the gas drum, which rises or moves down, according to the amount of gas stored. The gas drum is prevented from tilting by a guiding frame. • Easy to and to control operation • Material costs are high • High risk of corrosion and rusting (short lifespam).
Floating-drum Biogas Plants 3. Design Principals MUELLER (2007) MUELLER (2007)  Open gasholder Different design of floating drum plants   Floating drum plant with inlet from the the NGO BIOTECH (India)
Toilet linked Biogas Reactors •Co-digestion of toilet products (nightsoil or blackwater) is a sustainable solutions for • Hygienically safe on-site treatment of toilet excreta • Production of fertiliser • Production of renewable energy •The mixing of animal dung with blackwater increases its fluidity and results in optimal moisture conditions for the anaerobic digestion. •Human manure has a lower content in organic matter and thus a limited biogas yield. 3. Design Principals
92 Toilet linked Biogas Reactors 3. Design Principals Source: ???
3. Design Principals Collection and expansions chamber Gas outlet pipe Pour- flush toilet Link of toilet Inlet for animal waste Baffle to mix influent with tank contents Removable cover annual desludging Biogas reactor Toilet linked Biogas Reactors Source: adapted from WELL (n.a.)
Toilet linked Biogas Reactors 3. Design Principals Source: M. WAFLER Sludge drying bed Expansions chamber Biogas reactor Pour-flush toilet Manure and green waste mixing chamber http://www.ashdenawards.org/files/imagecache/large/fi les/images/biogasnepal05a.jpg [Accessed: 02.06.2010]
Health aspects •Anaerobic digested sludge are generally pathogen free. Pathogen removal depends temperature and retention time. Generally , at more than 55°C pathogens are killed after a few days. At normal temperatures (mesophilic digestion), longer time is required. 4. Treatment Efficiency Source: SASSE (1988) Source: WERNER et al. (1998) In reality, fresh sludge is always mixed with new sludge and it is very difficult to control retention times. Therefore, caution needs to be taken when emptying and handling sludge manually.
Nutrients Anaerobic digestion only removes organics, and the main mineral material and almost all nutrients remain in the bottom sludge. • Phosphorus: almost 100 % • Nitrogen (ammonium): and 50 to 70 % (JOENSSEN et al. 2004) ➔Biogas Slurry = Fertiliser Further treaments to increase the safety (pathogen removal) • Composting • Drying beds / Humification 4. Treatment Efficiency Biogas slurry = Fertilisers
Start-up •Seeding with living sludge form other anaerobic reactor required. The establishment of the complex biological conditions for anaerobic digestion and biogas production may takes some weeks to months. 5. Operation and Maintenance (O&M) Operation No skilled operator is required but households should be trained to understand the system. Regular maintenance includes • Checking for foaming or scum formation • Checking for air/gas- tightness • Checking for rusting (e.g. floating-drum reactor)
•Small-scale biogas digesters can transform almost any biodegradable waste into biogas. •Household or community scale. •Most often used for biogas production in rural areas from animal dung. •Green wastes (kitchen, garden, etc.) can be added. •If toilets are linked: safe and sustainable sanitation solution. •Underground construction provided: can also be constructed in urban areas. •As anaerobic digestion is limited to moderate to high temperature, only in areas where temperature does not fall short of for any substantial length of time. 6. Applicability
7. Pros’ and Cons’ Advantages: • Low-cost • Generation of biogas and fertilizer • Combined treatment of animal, human and solid organic waste • Low operation and maintenance • Underground construction (low space requirement and high acceptance) • Low risk of odours • Resistance against shock loads • Long life span if maintained and operated correctly • Reduces the amount of wood fuel and improves indoor air quality Disvantages: • Experts are required for the design of the reactor and skilled labour is required for the construction of a gastight tank • Substrates need to contain high amounts of organic matter for biogas production • Slurry may has to be further treated before reuse (e.g. composting) • Below temperatures of 15°C, biogas production is economically not interesting (heating required) • Requires seeding (start-up can be long due to the low growth yield of anaerobic bacteria)
Kinetics of anaerobic fermentation
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KINETICS OF ANAEROBIC FERMENTATION (Reference: Mital, pp 36-39):
Chen and Hashimoto, Biotechnology Bio-engineering Symposium 8, (1978) p 269-282
Most models used to describe biological waste treatment processes incorporate a bacterial decay in term in eq. (1) to account for disappearance of the bacterial mass through endogenous respiration and lysis. For a complete mixed system, the average solids retention time equals the hydraulic retention time.
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For a completely mixed, continuous-flow system under steady-state equations (4) –(7) gives:
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Methane production is directly correlated with COD reduction. Since no oxidizing agent is added, the only way COD reduction can occur is through the removal or organic material from the waste, such as through the evolution of methane and carbon dioxide. The other avenues of COD reduction through hydrogen sulfide and hydrogen gas evolution are insignificant.
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KINETICS OF DIGESTION Biotechnology Bioengineering (1982) 24: 9-23
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Fermentation Process: The term fermentation is often used interchangeably with anaerobic digestion when describing the physical decomposition of organic material (typically when discussing foods and beverages). In reality, fermentation is a distinct biological reaction that makes up one step in the greater process of anaerobic digestion. It is responsible for acidogenesis, the forming of acids. Fermentation is a metabolic pathway for certain microbial organisms in anoxic environments. During fermentation, larger organic molecules, like sugars, are converted into a mixture of reduced end products (products that have gained electrons). The process occurs in two steps (see diagram). First, energy (in the form of ATP molecules) is produced by the reactions of glycolysis, a process that breaks down sugars and converts them into pyruvate molecules. NAD+ molecules are used up in this step and are transformed into NADH. In the second step, NAD+ is recreated from NADH via oxidation and reduction reactions (which involve repositioning electrons). NADH molecules donate an electron to an acceptor. Because a typical substance that normally receives the electron, like oxygen, is not available, endogenous electron acceptors are utilized in this cycle. Pyruvate molecules, (created during glycolysis) accept the electron and are subsequently converted into substances such as acids and alcohols through further molecular rearrangement. Specific fermentation reactions differ according to the microorganism performing the process as well as the original substrates (sugars) being used. The result is the creation of varying end products. In the case of fermentation within anaerobic digestion, the production of a mixture of organic acids drive the decomposition process to create biogas.
Dry and wet fermentation
Diagram of Simplified Fermentation Process
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FLEXIBLE PORTABLE NEOPRENE RUBBER MODEL
HIGH RATE BIOGAS PLANTS FOR INDUSTRIAL WASTE WATER TREATMENT
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Anaerobic contact digester
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THANK YOU

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Module - 2.pdf

  • 1. Biogas Dr. Sourav Poddar Department of Chemical Engineering National Institute of Technology, Tiruchirappalli Tamil Nadu
  • 3. What is biogas? • Biogas is a methane rich flammable gas that results from the decomposition of organic waste material • Biogas is produced by anaerobic digestion or fermentation of biodegradable materials such as biomass, manure, sewage, municipal waste, green waste, plant material and energy crops. • Biogas also called as ‘Marsh gas’ • Biogas is a type of biofuel. • This type of biogas comprises primarily methane and carbon dioxide
  • 7. Advantages of biogas • Production of large amount of methane gas (ambient temperature storage) • Production of free flowing thick sludge • Odorless sludge • Sludge can be used as fertilizer and soil conditioner • Sanitary way for human and animal waste disposal • Conservation of scarce resources like wood
  • 8. Disadvantages • Explosion chances • High capital lost • Incorrect handling of liquid sludge causes pollution • Requires control and maintenance • Needs proper condition • Use as a fuel requires removal of CO2 and H2S
  • 9. Factors affecting yield and production of biogas Many factors affecting the fermentation process of organic substances under anaerobic condition are, • The quantity and nature of organic matter • The temperature • Acidity and alkanity (PH value) of substrate • The flow and dilution of material
  • 11. Biochemical reactions C 48.46 H 6.75 O 37.47 N 7.30 S 0.02 C16h27o9n2 + (27/2) h2 (23/2) ch4 + (9/2) co2 + h2 + 2 nh3 2C403h675o234n52s + (627) h2 (572) ch4 + (234) co2 + 104 nh3 + 2h2s
  • 12. Liquefaction by hydrolytic enzymes • Complex organic matter is degraded to basic structure by hydraulic bacteria. ➢Protein - Polypeptide and Amino Acid ➢Fat -Glycerin and Fatty Acid ➢Amylose - Monosacride and Polysacride
  • 13. Acid Formation • Also called the acidogenesis • Simple organic matters are converted into acetic acid, H2 and CO2 • Acting bacteria in this process are called hydrogen producing bacteria and acid-producing bacteria.
  • 14. Methane Production • Methanogenesis • In this process, acetic acid, H2, CO2, are converted into CH4. • Methane-producing bacteria have strict pH requirement and low adaptability to temperature. • Methanococcus jannaschii, Methanobacterium thermoautotrophicum
  • 15. Flow chart of anaerobic digestion
  • 16. Modes of operation • Batch • Semi-continuous – regular feeding of digester and decrease of organic matter at intervals • Continuous – for liquid waste treatment
  • 17. Continuous-fed System • Suited for large-scale manure substrate bioreactor. • Steady biogas production can be expected. • May require auxiliary equipments. • Requires high liquid content. • Temperature, loading rate, and solid content need to be carefully monitored.
  • 18. Batch-fed System • The simplest design. • Low cost. • The feedstock is loaded one batch at time. • Irregular biogas production. • Can operate on high solid content. • Requires manual labor.
  • 19. Types of digesters A typical biogas system consists of the following components: • (1) Manure collection • (2) Anaerobic digester • (3) Effluent storage • (4) Gas handling • (5) Gas use. TWO MAIN TYPES: 1. Digesters utilizing dispersed growth of bacteria 2. Digesters utilizing attached growth of bacteria
  • 20. DISPERSED GROWTH DIGESTERS 1. Fixed dome digester 2. Floating gas holder digester (Indian) 3. Plug flow digester (horizontal displacement) 4. Bag digester (Taiwan and Korea) 5. Separate gas holder digester 6. Conventional digester
  • 24. 1. Anaerobic filter 2. Up-flow anaerobic sludge blanket (UABSR) Attached growth digesters
  • 55. What is Anaerobic Digestion ? Degradation of organic material by bacteria. In the absence of air (anaerobic). Four stages: •Hydrolisis • Cleavage of a chemical compound through the reaction with water. • Insoluble complex molecules are bracken down to short sugars, fatty acids and amino acids. •Fermentation (Acidogenesis) • Products from hydrolysis are transformed into organic acids, alcohols, carbon dioxide (CO2), hydrogen (H) and ammonia (NH3). •Acetogenesis • Organic acids and alcohols are converted into hydrogen (H2), carbon dioxide (CO2) and acetic acid (CH3COOH). Therefore, oxygen is consumed and anaerobic conditions are created •Methanogenesis • Methanogenic bacteria (methanogenesis), transform the acetic acid, carbon dioxide and hydrogen into biogas. 1. Concept
  • 56. What is Anaerobic Digestion ? 1. Concept D. SPUHLER (2010)
  • 57. 1. Concept What is Anaerobic Digestion ? Source: http://water.me.vccs.edu/courses/ENV149/changes/Feat11_picII- 1.jpg [Accessed: 02.06.2010]
  • 58. What is Biogas ? •Biogas is a mixture of methane and carbon dioxide. •The properties of biogas are similar to the ones of natural gas. •Biogas is the common name for the mixture of gases released from anaerobic digestion. •Typically biogas is composed of: •Methane is the valuable part of the biogas. Biogas that contains about 60 to 70 % of CH4 has a calorific value of about 6 kWh/m3 what corresponds to about half an L of diesel oil. 1. Concept Sources: YADAV & HESSE (1981); FAO (1996); PIPOLI (2005); GTZ (2009 Methane (CH4) 50 to 75 % Carbon Dioxide (CO2) 25 to 50 % Hydrogen (H) 5 to 10 % Nitrogen (N2) 1 to 2 % Hydrogen sulphide (H2S) Traces Source: MUENCH (2008)
  • 59. Anaerobic Digestion of Green Waste 1. Concept D. SPUHLER (2010), Pictures from: www.kristianstad.se/; http://www.newseedadvisors.com/2009/09/10/invest/; http://www.hydroharrys.com/hydroharrys_about_fertilizer.php; http://www.mytinyplot.co.uk/advice/the-art-of-composting/ www.clker.com [Accessed: 02.06.2010] Green Waste Coocking Lightning Electricity Fuel Biogas Heating Fertiliser Agriculture
  • 60. Anaerobic Digestion of Green Waste: Small- scale 1. Concept Biogas Soil amendement Food Energy D. SPUHLER (2010). Pictures from: //gardening.ygoy.com/wp-content/uploads/2009/10/how-to-plant-a-decorative-vegetable- garden0.jpg; http://www.gardenplansireland.com/forum/about436.html; http://www.clker.com/ [Accessed: 06.06.2010]
  • 61. Anaerobic Digestion of Green Waste: Large-scale 1. Concept Source: HOLLIGER (2008)
  • 62. 1. Concept Co-digestion of waste and manure Food production at farms Re-use of nutrients Re-use of energy The example of Kristianstad (Sweden) Green waste from households and industries
  • 63. What is Green Waste? 1. Concept Kitchen refuses http://www.mytinyplot.co.uk/advice/the-art-of- composting/ [Accessed: 04.06.2010] http://www.ceroi.net/reports/dushanbe/eng/ waste.htm [Accessed: 04.06.2010] Organic fraction of municipal waste http://www.agro- resources.com/uploads/images /chocolate%20waste.jpg [Accessed: 04.06.2010] Refuses from the food industry http://www.titech.com/assets/x/50186 ?width=82 [Accessed: 04.06.2010] Some industrial wastes http://www.bawbawshire.vic.gov.au/Page/i mages/green-waste-grass.jpg [Accessed: 04.06.2010] Garden refuses Waste from agriculture http://planetgreen.discovery.com/hom e-garden/images/2009-04/organic- waste.jpg [Accessed: 04.06.2010] Market waste http://www.ducorwaste.org/images/Rally_Time_ Stockpike.jpg [Accessed: 04.06.2010] •Green waste is any kind refused material which is biodegradable and has a high fraction of organic matter, which can be transformed into biogas. Some examples…
  • 64. Examples: Small-scale Biogas plants 1. Concept Biogas plant for cow dung, Padli village (India) Source: M. WRIGHT, Ashden Awards Source: M. WRIGHT, Ashden Awards Source: M. WRIGHT, Ashden Awards
  • 65. Examples: Small-scale Biogas Plants 1. Concept Biogas lamp Source: M. WRIGHT, Ashden Awards Source: M. WRIGHT, Ashden Awards Adding greywater to the biogas reactor to optimise moisture conditionss
  • 66. Examples Small-scale Biogas Plants 1. Concept The “Mudbooster” Plant Source: UNKNOWN
  • 67. Examples Small-scale Biogas Plants 1. Concept Source: C. RIECK (2009) The manhole is filled with water to keep the clay sealing wet and gas tight. Gas leackage would be indicated by bubbles. Source: C. RIECK (2009) Wet clay is used to fit the concrete lid of the manhole gas-tight. Source: SuSanA Biogas outlet and manhole with remouvable cover from a underground biogas plant Installed by the NGO TED in Maseru, Lesotho (Susana)
  • 68. 2. How it can optimize SSWM D. SPUHLER (2010), adapted from: http://www.terranet.or.id/mitra/dewats/photo/masukan1256.jpg; http://www.borda-sea.org/modules/cjaycontent/index.php?id=6; http://whrefresh.com/wp- content/uploads/2010/01/potato_field.jpg; http://www.greenspec.co.uk/images/energy/CHP/chp2.gif]; http://peda.gov.in/eng/images/rural- biogas-plant_179.jpg; [Accessed: 30.05.2010], BPO (2006) and BUNNY (n.y.) Biogas plants transform traditional manure management; reducing CH4 and CO2 emission Biogas substitutes conventional energy sources, reducing reliance on fossil fuel and firewood (CO2) Digested sludge can substitute chemical fertiliser Biogas plants can contribute to sustainable sanitation
  • 69. 2. How the Digestion of Green Waste optimises SSWM Digested sludge can substitute chemical fertiliser and enhance food production Green Waste Coocking Lightning Electricity Fuel Biogas Heating Fertiliser Agriculture Sustainable development: • Improved health • Improved economy Anaerobic digestion is a promising answer to the soaring crises of municipal waste explosion and thus prevent the pollution of water sources and the evnvironment Biogas is an renewable energy and has the potential to replace other fuel sources. Biogas contributes to prevent and lower greenhouse gas emission.
  • 70. Examples: Biogas Appliances 1. Concept Chang Mai K.P . Pravinjith M. Wafler Biogas generator Biogas rice cooker Biogas boiler Biogas lamps Biogas cooking stoves PBO (2006) Krämer (TBW) Source: UNKNOWN
  • 71. The ARTI compact biogas plant •Developed in 2003 •ARTI = Appropriate Rural Technology Institute •2000 plants currently used in Maharashtra, India (WRAPAI 2009) •Some have been constructed in Tanzania (VOEGELI & LOHRI 2009) •Floating-drum design: •2 conventional polyethylene tanks (0.75 and 1 m3). (MUELLER 2007) •Standard plumber piping. •The smaller tank is the gasholder and the larger holds the mixture of decomposing feedstock and water •Inlet and an overflow •Overflow liquid is mixed with the feedstock and back recycled into the plant to maintain optimal moisture condition. (MUELLER 2007) •A pipe takes the biogas to a collection balloon or directly to the kitchen. Examples of Applications Source: HEEB (2009) Source: VOEGELI & LOHRI (2009)
  • 72. The BIOTECH Plant •BIOTECH is a nodal agency of the Ministry of Non-Conventional Energy Sources in Kerala, South India. (MUELLER 2007) •Domestic plants: 1 m3 for a 3 to 5 member-family meets about 50 % of cooking needs. (MUELLER 2007) •Decentralised treatment of market waste, municipal solid waste or slaughterhouse waste: Biogas used for public street lightning and distributed into households Examples of Applications Source: HEEB (2009) Source: HEEB (2009) Source: HEEB (2009)
  • 73. Source: HEEB (2009) The BIOTECH Plant •Schematic plan of a BIOTECH market level plant. a) Inlet tank for feedstock. b) Digester tank. c) Effluent tank. d) Effluent storage tank. e) Effluent pump. f) Gasholder drum. The drum is stabilized by a guide pole in the middle and is floating in a water jacket outside the digester. g) Biogas pipe. h) Gas Scrubber. i) Biogas generator j) Drainage connection for excess effluent. (HEEB 2009) Examples of Applications • Conventional floating-drum reactor • Liquids are mixed with incoming feedstock and re-circulated • A baffle in the middle increases solids retention
  • 74. The KOMPOGAS Plant •Thermophilic dry digestion process developed in Switzerland •Organic wastes come from municipalities with source separation or from food industry •Horizontal plug-flow reactors •Propellers move the sludge trough the reactor and keep particles in suspension. (OSTREM 2004) •Retention time is 20 days. (MES et al. 2003) Examples of Applications Biogas is transformed in a CHP unit providing 100 % of the facility needs as well as additional electricity for sale. In some cases, the biogas is upgraded to natural gas standards for use in vehicles or input to the natural gas network. (OSTREM 2004) Both liquid and solid effluents are commercialised fertilisers
  • 75. Large-scale Plant in Thailand for Municipal Waste •In Thailand, where the development of alternative sources is critical to energy sustainability as the government has set 2011 as the target date for 80 % of the nation’s total energy, representing an estimated 1,900 MW, to be generated from renewable energy sources. (MUELLER 2007) •This has given large rise to various large-scale biogas projects. The Rayong Municipality has constructed plant for the treatment of the organic fraction of the municipal solid waste (MSW) with a capacity of 60 tons of waste per day. •Two systems: •Digesters: converts waste to biogas and fertilizer •A biogas-fired cogeneration process (CHP) Examples of Applications MUELLER (2007) MUELLER (2007)
  • 76. •Process: Wet fed-batch high-solids digestion •Feedstock: Organic MSW and refuses from the food industry. •Capacity: 60 tons per day •Output: 5800 tons organic fertilizer and electricity of about 5 million kWh.(MUELLER 2007) Large-scale Plant in Thailand for Municipal Waste Examples of Applications SourcE: MUELLER (2007)
  • 77. Basics: Process Parameters The biogas yield depends on the process and the substrate. Substrate: •High COD (Chemical Oxygen Demand) = High potential of biogas generation Process: Anaerobic digestion = Biological system of bacteria Optimal conditions required that bacteria feel wealthy… •Temperature - Temperature is an important parameter. Mesophilic methane producing bacteria grow at an optimum temperature of 35oC the gas production rate drops very much when temperature is less than 10oC. •Performance •Retention time - The ratio of volume of slurry in the digester to the volume fed into and removed from it per day is called retention time. Thus a 20 liter digester is fed at 4 liters per day so that the volume of digester is constant the retention time is 5 days. The required retention time is normally 30 days for mesophilic (25-35oC) conditions. •pH • Wide range, but methanogenesis requires neutrality (6.5-7.5) (MES et al. 2003) • Multistage process for better pH and temperature control •Total solid (TS) and moisture • Wet digestion (TS < 20 %): easier to maintain, good fluidity • Dry digestion (TS > 20 %): sophisticated but safes space • Solids for digestion (organics) - Liquid for fluidity of slurry. • Optimal TScontent: 5 to 10%. (SASSE 1988; NIJAGUNA 2002) Design Principals
  • 78. Basics: Process Parameters Anaerobic digestion = Biological system of bacteria Optimal conditions required that bacteria feel wealthy… • COD: Chemical oxygen demand: Methane production potential • Ratio of carbon to nitrogen in the waste water influent or C/N ratio is 30:1 and if nitrogen content in ammoniacal form is less the bacterial growth is affected and the process slows down. • Volumetric organic loading rate: This can be expressed as kg Vs per volume per day based on the % weight of organic matter added each day to the digester volume. • Digester loading rate % = (Per cent of organic matter in feed)/(Retention Time) Loading rate range is 0.7 to25 kg VS/ m3 / Day Design Principals
  • 79. • Anaerobic digestion can transform almost any biodegradable waste into biogas (e.g. green waste). •The anaerobic treatment of organic solid waste is applicable everywhere where there is a need for biogas and waste treatment and the technical conditions allow the installation of a plant. •Small-scale (biogas generation for cooking and lightening) – low-cost and relatively low-tech: • Household-level • Community-level • Institutional-level •Large-scale – high-tech, requires expert design: • Industrial plant connected to the public power and heat grid. •Low-tech (un-heated plants), however, are only adapted to areas where temperature does not fall short of for any substantial length of time. Applicability
  • 80. Basics: Daily manure yield for different cattle Design Principals Sources: OEKOTOP; WERNER et al. (1998)
  • 81. Basics: Gas yields for different feedstocks Design Principals Sources: OEKOTOP; WERNER et al. (1998)
  • 82. Basics: Biogas Guideline data Design Principals Adapted from WERNER et al. (1998); ISAT/GTZ (1999), Vol. I; MANG (2005) Suitable digesting temperature 20 to 35 °C Retention time 40 to 100 days Biogas energy 6kWh/m3 = 0.61 L diesel fuel Biogas generation 0.3 – 0.5 m3 gas/m3 digester volume per day Human yields 0.02 m3/person per day Cow yields 0.4 m3/Kg dung Gas requirement for cooking 0.3 to 0.9 m3/person per day Gas requirement for one lamp 0.1 to 0.15m3/h
  • 83. Types of Digester: Bag or Rubber Balloon Biogas Plants Design Principals Batch mode: emptying once every few years Plug-flow reactor: the slurry moves through continuously much like a train a tunnel •Huge common plastic bag (e.g. PVC): sludge settles on the bottom and biogas is collected in the top. Gas is transported by the pressure from the elasticity of the balloon (can be enhanced by placing weights on the balloon). • Most simple design, easy and low-cost ( if material locally available) • Temperature enhanced when exposed to sun • Simple to clean but lifespan generally limited Plastic bag Layer of compacted backfill To reuse or further treatment (e.g. drying bed) Source: adapted from FAO (1996) Gas pipe Inlet Leveled surface Biogas accumulates in the top of the bag
  • 84. Types of Digester: Bag or Rubber Balloon Biogas Plants Design Principals Source: http://www.habmigern2003.info/biogas/Baron-digester/Baron-digester.htm [Accessed: 02.06.2010] Underground plug- flow reactor bag biogas plant () and balloon biogas collection chamber (). (Philippines, Garry Baron)
  • 85. Types of Digester: Fixed-dome Biogas Plants •Airtight underground reactor out of concrete or brick work (most often round), with a fixed (also airtight) dome in which gas is collected. Gas pressure is absorbed by the slurry which is displaced into a compensation tank. • Most widely disseminated • Long life-spam • Underground: safes space and protect from temperature changes • Construction must be supervised Design Principals
  • 86. Types of Digester: Fixed-dome Biogas Plants Design Principals Removable cover Overflow tank / compensation chamber Seal Slurry Biogas accumulates in the dome Biogas collection Inlet Source: adapted from http://peda.gov.in/eng/images/rural-biogas-plant_179.jpg [Accessed: 02.06.2010] Fixed- dome
  • 87. Source: K.P. PRAVVIJITH Source: K.P. PRAVVIJITH Source: K.P. PRAVVIJITH Source: K.P. PRAVVIJITH Types of Digester: Fixed-dome Biogas Plants Design Principals
  • 88. Plastic dome Design Principals Any pit can be filled with organic waste and covered airtight with a plastic sheet in order to collect biogas Source: ISAT/GTZ (1999, Vol. I)
  • 89. Floating-drum Biogas Plants 3. Design Principals Floating- drum Biogas Slurr y Inle t Outl et •Floating-drum plants consist of an underground digester and a moving gasholder (mostly of made out of steel). The gasholder floats either directly on the fermentation slurry or in a water jacket of its own. The gas is collected in the gas drum, which rises or moves down, according to the amount of gas stored. The gas drum is prevented from tilting by a guiding frame. • Easy to and to control operation • Material costs are high • High risk of corrosion and rusting (short lifespam).
  • 90. Floating-drum Biogas Plants 3. Design Principals MUELLER (2007) MUELLER (2007)  Open gasholder Different design of floating drum plants   Floating drum plant with inlet from the the NGO BIOTECH (India)
  • 91. Toilet linked Biogas Reactors •Co-digestion of toilet products (nightsoil or blackwater) is a sustainable solutions for • Hygienically safe on-site treatment of toilet excreta • Production of fertiliser • Production of renewable energy •The mixing of animal dung with blackwater increases its fluidity and results in optimal moisture conditions for the anaerobic digestion. •Human manure has a lower content in organic matter and thus a limited biogas yield. 3. Design Principals
  • 92. 92 Toilet linked Biogas Reactors 3. Design Principals Source: ???
  • 93. 3. Design Principals Collection and expansions chamber Gas outlet pipe Pour- flush toilet Link of toilet Inlet for animal waste Baffle to mix influent with tank contents Removable cover annual desludging Biogas reactor Toilet linked Biogas Reactors Source: adapted from WELL (n.a.)
  • 94. Toilet linked Biogas Reactors 3. Design Principals Source: M. WAFLER Sludge drying bed Expansions chamber Biogas reactor Pour-flush toilet Manure and green waste mixing chamber http://www.ashdenawards.org/files/imagecache/large/fi les/images/biogasnepal05a.jpg [Accessed: 02.06.2010]
  • 95. Health aspects •Anaerobic digested sludge are generally pathogen free. Pathogen removal depends temperature and retention time. Generally , at more than 55°C pathogens are killed after a few days. At normal temperatures (mesophilic digestion), longer time is required. 4. Treatment Efficiency Source: SASSE (1988) Source: WERNER et al. (1998) In reality, fresh sludge is always mixed with new sludge and it is very difficult to control retention times. Therefore, caution needs to be taken when emptying and handling sludge manually.
  • 96. Nutrients Anaerobic digestion only removes organics, and the main mineral material and almost all nutrients remain in the bottom sludge. • Phosphorus: almost 100 % • Nitrogen (ammonium): and 50 to 70 % (JOENSSEN et al. 2004) ➔Biogas Slurry = Fertiliser Further treaments to increase the safety (pathogen removal) • Composting • Drying beds / Humification 4. Treatment Efficiency Biogas slurry = Fertilisers
  • 97. Start-up •Seeding with living sludge form other anaerobic reactor required. The establishment of the complex biological conditions for anaerobic digestion and biogas production may takes some weeks to months. 5. Operation and Maintenance (O&M) Operation No skilled operator is required but households should be trained to understand the system. Regular maintenance includes • Checking for foaming or scum formation • Checking for air/gas- tightness • Checking for rusting (e.g. floating-drum reactor)
  • 98. •Small-scale biogas digesters can transform almost any biodegradable waste into biogas. •Household or community scale. •Most often used for biogas production in rural areas from animal dung. •Green wastes (kitchen, garden, etc.) can be added. •If toilets are linked: safe and sustainable sanitation solution. •Underground construction provided: can also be constructed in urban areas. •As anaerobic digestion is limited to moderate to high temperature, only in areas where temperature does not fall short of for any substantial length of time. 6. Applicability
  • 99. 7. Pros’ and Cons’ Advantages: • Low-cost • Generation of biogas and fertilizer • Combined treatment of animal, human and solid organic waste • Low operation and maintenance • Underground construction (low space requirement and high acceptance) • Low risk of odours • Resistance against shock loads • Long life span if maintained and operated correctly • Reduces the amount of wood fuel and improves indoor air quality Disvantages: • Experts are required for the design of the reactor and skilled labour is required for the construction of a gastight tank • Substrates need to contain high amounts of organic matter for biogas production • Slurry may has to be further treated before reuse (e.g. composting) • Below temperatures of 15°C, biogas production is economically not interesting (heating required) • Requires seeding (start-up can be long due to the low growth yield of anaerobic bacteria)
  • 100. Kinetics of anaerobic fermentation
  • 103. KINETICS OF ANAEROBIC FERMENTATION (Reference: Mital, pp 36-39):
  • 104. Chen and Hashimoto, Biotechnology Bio-engineering Symposium 8, (1978) p 269-282
  • 105. Most models used to describe biological waste treatment processes incorporate a bacterial decay in term in eq. (1) to account for disappearance of the bacterial mass through endogenous respiration and lysis. For a complete mixed system, the average solids retention time equals the hydraulic retention time.
  • 107. For a completely mixed, continuous-flow system under steady-state equations (4) –(7) gives:
  • 109. Methane production is directly correlated with COD reduction. Since no oxidizing agent is added, the only way COD reduction can occur is through the removal or organic material from the waste, such as through the evolution of methane and carbon dioxide. The other avenues of COD reduction through hydrogen sulfide and hydrogen gas evolution are insignificant.
  • 112. KINETICS OF DIGESTION Biotechnology Bioengineering (1982) 24: 9-23
  • 115. Fermentation Process: The term fermentation is often used interchangeably with anaerobic digestion when describing the physical decomposition of organic material (typically when discussing foods and beverages). In reality, fermentation is a distinct biological reaction that makes up one step in the greater process of anaerobic digestion. It is responsible for acidogenesis, the forming of acids. Fermentation is a metabolic pathway for certain microbial organisms in anoxic environments. During fermentation, larger organic molecules, like sugars, are converted into a mixture of reduced end products (products that have gained electrons). The process occurs in two steps (see diagram). First, energy (in the form of ATP molecules) is produced by the reactions of glycolysis, a process that breaks down sugars and converts them into pyruvate molecules. NAD+ molecules are used up in this step and are transformed into NADH. In the second step, NAD+ is recreated from NADH via oxidation and reduction reactions (which involve repositioning electrons). NADH molecules donate an electron to an acceptor. Because a typical substance that normally receives the electron, like oxygen, is not available, endogenous electron acceptors are utilized in this cycle. Pyruvate molecules, (created during glycolysis) accept the electron and are subsequently converted into substances such as acids and alcohols through further molecular rearrangement. Specific fermentation reactions differ according to the microorganism performing the process as well as the original substrates (sugars) being used. The result is the creation of varying end products. In the case of fermentation within anaerobic digestion, the production of a mixture of organic acids drive the decomposition process to create biogas.
  • 116. Dry and wet fermentation
  • 117. Diagram of Simplified Fermentation Process
  • 135. FLEXIBLE PORTABLE NEOPRENE RUBBER MODEL
  • 136. HIGH RATE BIOGAS PLANTS FOR INDUSTRIAL WASTE WATER TREATMENT